Debug logs for Site Guest User

Please follow below steps

1. Ask the user to set a browser cookie with a domain of .force.com, a name of debug_logs, and any value. Refer to the documentation for your user’s browser for information on adding cookies. To add cookies, your user probably needs a browser plug-in or extension for web development.
   • To set a cookie for API requests made with Java code, use the URLConnection class and set the cookie value as follows.

     URL url = new URL("http://yourSite.force.com/");
     URLConnection con = url.openConnection();
     con.setDoOutput(true);
     con.setRequestProperty("Cookie", "debug_logs=debug_logs,domain=.force.com");
     con.setRequestProperty("Content-Type", "text/plain; charset=utf-8");
     con.connect();

   • To set a browser cookie in Google Chrome™:
     1. Navigate to your site.
     2. Open the Chrome DevTools Console by pressing Ctrl+Shift+J (Cmd+Opt+J on macOS).
     3. Execute this command.

        document.cookie="debug_logs=debug_logs;domain=.force.com";

   • To set a browser cookie in other browsers, install a plug-in or extension.
2. Find the name of your site’s guest user.
   1. From Setup, enter Sites in the Quick Find box, then select Sites.
   2. Select your site from the Site Label column.
   3. Select Public Access Settings | View Users.
3. Set a user-based trace flag on the guest user.
1. From Setup, enter Debug Logs in the Quick Find box, then click Debug Logs.
2. Click New.
3. Set the traced entity type to User.
4. Open the lookup for the Traced Entity Name field, and then find and select your guest user.
5. Assign a debug level to your trace flag.
6. Click Save.

Reference Link : https://help.salesforce.com/apex/HTViewSolution?id=000247187&language=en_US